Ergo Final, why me?
Task requirements for work station
- Work height vs work surface height -Normal area vs maximum area (object location) -Slanted surfaces
Major seating issues
-Available standards are many, too general, and not task specific -Wide latitude in recommended standards (usually population based, no one best posture or postures) -Multiple evaluation criteria and influences (Duration of seated work, sit vs stand ratio, reach envelopes and visual requirements, preferred postures) -Interpretation of subjective criteria (30-60 min required for stabilization of ratings, short term evaluations sell; long term comfort and health unknown, and level of discomfort) -Lack of good functional (preferred) seating anthropometry and postures -Multiple outcome measures used; limited interpretation (Discomfort, EMG, disc pressure Reach and vision performance, skin temperature and pressure) -Training needed in using available adjustments -Many new designs currently offered and promoted -Integrating seats with work surfaces, controls, and displays
Spine Biomechanics
-Functionally four parts -Two mobile parts: cervical and lumbar spine -Relatively immobile thoracic spine -Sacrum
Advantages of standing work?
-Greater reach capacity (larger work envelope) -Biomechanical advantage (more leverage) -Legs dampen vibration -Lower disc pressure (only when not flexed) -Increased trunk power (we can work harder) -Less leg room required (horizontally)
Disadvantages of standing work
-Greater reach capacity, larger work envelope (facilitates greater postural stress) -Biomechanical advantage, more leverage (Facilitates greater stress) -We have a greater work capacity when standing therefore leaves the opportunity for greater task demand (fatigue, high joint loads)
How does lower limb posture affect pelvic rotation
-Hamstring muscles cross both the knee and hip joints -Two-joint effect (dependence of knee and hip ROM) -Lower limb posture can influence pelvis posture -Combined knee extension and hip joint flexion: (elongation of hamstring muscles; large, passive, muscle forces; and resultant moment acting to rotate pelvis posteriorly -Knee flexion and hip extension can cause anterior pelvis rotation and lumbar hyper-lordosis
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-Inclusion of lumbar support reduces intradiscal pressure for any given seat inclination -Maintenance of normal lordotic curve -Minimize 'wedging' of intervertebral disc caused by spine flexion -Disc wedging causes higher pressure for a given axial load (Fc) -Vertical backrest inclinations (>=degree) reduce intradiscal pressure for any given lumbar support size -Inclination allows for extension of the lumbar spine -Inclination increases magnitude of BW supported by the backrest -Use of arm rests decreases disc pressure for any back rest inclination -EMG data consistent with intra-discal pressure, though some indications that there can be too much lumbar support
Common horizontal measures in seated anthropometry?
-Internal and external sitting depth -Abdominal depth
Controlling for static exertions
-Interspersed rest periods can increase endurance times -Rest periods, though, will be relatively ineffective for strenuous exertions (80-100% MVE) -Minimize joint moments. Even small extremity deviations will result in high moment loads. -Minimize reach distances to minimize joint moments. -Maximize strength by keeping objects close to the body -Minimize head tilt to decrease neck fatigue -Maximize capacity by using larger muscle groups and postures that are near the middle of the range-of-motion (length-tension relationship)
WS Design Summary
-Issues in seat design include maintaining a lordosis of the spine, and effectively distributing forces and pressures -Good work station design attempts to reduce or eliminate localized muscle fatigue -Improper work heights can cause localized muscle fatigue in the shoulder -The moment created by the head about the neck can create disc compression problems similar to the lower back -Designs that minimize joint loads, also minimize fatigue, maximize endurance, and maximize performance -A need exists for ergonomic analysis methods and guidelines for fatigue in 'realistic' task conditions
Spinal problems in standing posture?
-Low back pain is very common (In postures with pronounced lumbar lordosis, the facet joints may begin to take on some of the compressive load; in presence of forward inclination of trunk while working) -Excessive facet joint loading may be causal factor in the incidence of osteoarthritis -Stress on the soft tissue around the joint -Spine hyperextension might be beneficial in temporary decreasing the load on the intervertebral disk
Considerations in designing work stations
-Need to carefully consider posture especially for static work components and load handling -Among the variables that need attention are: (Work heights: Objects are best placed between hip and shoulder height; Reaches; Loads handled; Static Postures) -No complete guidelines or "rules of thumb" for workstation design except that work surfaces and seats should be adjustable -A good workstation puts the worker in a good work posture (one that allows for efficient work with a minimum amount of discomfort)
Advantages of RULA?
-No equipment expenditures -Can be performed in confined spaces with no/little disruption of the task -no previous experience in observational methods (though helpful) -developed to allow for quick assessments and initial recommendations (conservative estimate)
Strength and performance depend on workstation layout
-Push in and pull out forces are lowest when the hand is close to the body -Left and right push and pull forces are lowest when the hand is far from the body -Workstation should consider strength 'zones' -Often specific mechanical requirements can best be achieved in specific postures -Design to minimize joint loads, also minimizes fatigue, maximizes endurance, and maximizes performance
What is relative risk (measures of exposure-disease association)
-Risk in exposed group divided by risk in unexposed group -X times the risk of developing disease, given exposure to the risk factor
What are the advantages of REBA?
-Sensitive to musculoskeletal risks in a variety of tasks -Assessment based on body segments which are evaluated individually -Include coupling for handling loads -Scores dynamic, static, and rapid, unstable postures -Action levels for prioritizing redesign/investigation efforts
Common vertical measures of seated anthropometry
-Sitting height -Elbow, thigh, patellar, shoulder, eye height
Some reasons why we need to move
-Static loads decrease blood flow to muscle (substantial decrease at > 15-20% MVC) -Loads as low as 4-6% MVC can cause fatigue -More rapid onset of fatigue with static muscle exertions as compared to equivalent dynamic exertions -Prolonged standing can cause blood pooling in legs
Relationship of shoulder muscles and chair/table height
-Table height is not only about lower back, but shoulder and upper torso will be affected -The need to support the upper extremities is determined by the relative heights of the person, chair, and table as well as the work position
What can be said about head and arm static exertions during seated work
-Time to reach shoulder muscle fatigue will be shorter with increased shoulder flexion and increased hand-held weights -More rapid fatigue onset with increasing shoulder abduction, especially greater than 30 degrees -Shorter endurance times associated with increased forward reach distances -Even with elbows supported by chair, fatigue time is still affected by reach distance -Increasing forward tilt of the head (especially greater than 30 degrees) will cause earlier fatigue onset
Characteristics of psychophysical methods
-User ratings (adjective and visual scales) -Best method for user evaluation? -Discomfort ratings -Questionable precision and repeatablity
Physiology of standing
-Very little increase of energy expenditure by changing from supine to standing posture (~8%) -Peak plantar (Foot) pressure of 137 kPA exceed the normal systolic pressure pressure of 17kPa (Occlusion of blood flow through the foot, walking can decrease the pressure) -Prolonged standing will result in circulatory insufficiencies and discomfort in lower extremety -Prolonged standing (peripheral pooling of blood, decrease in stroke volume and increase in heart rate) -Constrained standing (Troublesome particularly for older workers or those with peripheral vascular disease; Fidgeting is a preconscious defense against the postural stresses of constrained standing or sitting)
Minimizing muscle fatigue
-Work-rest cycles (Short and frequent is best) -Decrease magnitudes of static or sustained loads (Decreased moments -> decreased muscle forces; Work upright and keep objects close) -Support body segments (Arm rests; Lumbar support; Tool support, holsters, or balancing) -Floor mats (Some evidence that there is increased blood flow to legs, less impact, decreased perceived fatigue, and decreased leg swelling)
Causes of postural stress in standing?
-Working height of the hands is too high (Extension in the low back; excessive lordosis) -Work surface is too low (Trunk flexion, back muscle strain) -Lack of toe clearance (Increase horizontal distance; moment arm) -Asymmetric or "non-neutral" postural constraints (Eliminate fixity, if not possible, then make work posture as natural as possible) -Asymmetrical loading/working away from the midline of the body (Twisting makes the spine more prone to injury)
Methods of measuring comfort
1) Observation of the body postures and movements 2) Observation of the performance of the task 3) Direct objective ratings of general comfort checking off features of chairs on checklist, body area comfort rankings, and general comfort rankings.
How many zones can the shoulder angle be divided into?
3 (acceptable, conditionally acceptable, and not acceptable)
In normal standing postures, how far is the pelvis tilted anteriorly?
30-45 degrees
What is the maximum duration for stoop posture, or those at the extremes of reach?
3less than 1 min
What is the Odds Ratio (OR)?
A statistic that describe the likelihood of an outcome given exposure to a risk factor Used when subjects are selected on the basis of their disease status X times the risk of being exposed, given that the illness/injury is present
What is the rapid upper limb assessment (RULA)
A survey method for ergonomic investigation where upper limb disorders are of concern RULA is a screening tool that assesses biomechanical and postural loading on the whole body with particular attention to the neck, trunk and upper limbs. A RULA assessment requires little time to complete and the scoring generates an action list which indicated the level of intervention required to reduce the risks of injury due to physical loading on the operator. RULA is intended to be used as part of a broader ergonomic study. **Not meant to be used independently as the only analysis tool Represent a moment in the work cycle (postures selected for evaluation may be those that are held for the longest period of time or what appears to be the worst posture assumed; can assess one or both limbs) Posture categories are based on literature Scoring based on expert opinion of biomechanical and muscle function criteria Emphasizes careful delineation of upper extremity postures, as well as classifying the neck, trunk, and lower extremity postures Original paper provided rules and drawings to assist the job analyst in appropriately classifying the observed postures, and they also provided guidance as to acceptable and unacceptable postures
Which shoulder muscles are responsible for upper arm posture?
Abduction: middle deltoid, supraspinatus (C) -Flexion: deltoid, pectoralis -Internal/external rotation: pectoralis, infraspinatus -Work surface much below elbow: trapezius (B)
What is the coding system of RULA?
Action list level of required intervention
Pros and Cons of Seated work
Advantages: Greater stability for tasks requiring high visual and motor control, less energy consumption than standing, less stress on the lower extremities, lower hydrostatic pressure in the lower extremities Disadvantages: Some evidence to indicate an increased incidence of low-back pain for seated work
Why is the pressure lowest when the sitting with the back straight
An increase in the trunk load moment when the pelvis is rotated backward and the lumbar spine and torso are rotated forward The deformation of the disc itself caused by lumbar spine flattening
Benefits of Compression hosiery
Assist in venous return (circulation), proven to significantly reduce leg swelling, and decrease in reporting of discomfort/pain
14 Guidelines of workstation design
Avoid static loads and fixed work postures, reduce cumulative trauma disorder risks, work height at ~50 mm below elbow, give employee an adjustable chair, use feet as well as hands, use gravity (don't oppose it), conserve momentum, use two-handed motions rather than one-handed, use parallel motions for eye control of two-handed motions, use rowing motions for two-handed motions, pivot motions about the elbow, use the preferred hand, keep arm motions in the normal work area, let the small woman reach; let the large man fit
What is the postural adaptation with decreased toe clearance?
Bending forward -Pelvic tilting and lumbar flexion -more stress on the spine
What are the risk assessment methods?
Biomechanical Analysis, NIOSH WPG, Energy expenditure, Checklist/Tools (Posture Evaluations, place evaluations, lighting), & Work ability
Key concepts of seated work?
Biomechanics, Anthropometry, lumbar lordosis, disc pressure, legs, head and arms, shoulders, static muscle loading, fixed posture, adjustability, task requirements
How does OWAS work?
Breaks down work into phases (postures) Each phase is analyzed for: Posture of the back (1-4), posture of the arms (1-3), posture of the legs (1-7), load (use of force, 1-3) Each posture score is in 1 of 4 categories Categories 3 & 4 warrant action
Common transverse measures in seated anthropometry
Buttocks, shoulder, external elbow width
Footwear and standing aids
Choices of shoes can affect both comfort and safety -Massai barefoot technology, wobble shoe, and constant instability (changes direction of loads)
What do anti-fatigue mats provide?
Comfortable, safe interface between between feet and floor -Hard floors cause standing fatigue; carpet is least fatiguing of all major surfaces, concrete is one the worst
What is prevalence (Measures of disease frequency)?
Count of existing and new cases divided by total persons
What is Rapid Entire Body Assessment (REBA) designed for?
Designed to assess various unpredictable working postures found in health care and other service industries Similar applications and limitations
What are the goals of risk assessment?
Determine the absolute or relative risk of injury associated with a job or task (quickly and efficiently) Job Screening (Identify at risk workers; highlight high risk job methods or tasks) Record keeping (Baseline for evaluation of interventions)
What areas can OWAS be applied?
Development of a workplace or a work method, to reduce its musculoskeletal load and to make it safer and more productive Planning of a new workplace or work method Ergonomic surveys Occupational health surveys Research and development
Considerable points in the disc pressure diagram
Disc pressure is considerably lower in standing than in unsupported sitting In different unsupported sitting postures, the lowest pressure was found when subject was sitting with the back straight
What are different visual demands
Display, Horizontal line of sight, resting position of eyes, seated reading standing, and seated work
What body part does the strain index consider?
Distal Upper extremity (elbow to hand)
What are the advantages of checklists?
Easy to administer, relatively inexpensive, gives snapshot in time, tools designed to be used in the workplace, and many tools to choose from
Critical dimensions of seating biomechanics
Elbow height (rest position = 90 degree) Popliteal height Thigh clearance Work height
What does the strain index combine?
Existing knowledge and theory regarding the physiology, biomechanics, and epidemiology (injury statistics) of distal upper extremity WMSDs (hand/wrist)
Disadvantages of checklists?
Expert knowledge needed, fuzzy lines between classifying high/low risk tasks, subjective measures depend on rater, can require lots of time and resources, few tools have been formally evaluated or validated
Characteristics of strain index?
Expert observations -Requires training -Very time consuming Resources -Knowledge of time-motion studies -Multiple raters -Stopwatches and video
What are the basic concepts used in epidemiology?
Exposure - the condition of being subjected to something be it an infectious agent, extremes of weather, or mechanical loading Outcome - a change in personal condition, normally as a result of 1+ exposures Case - an individual who has developed the outcome of interest
Understanding of Epidemiology (slide 5)
Exposures lead to Outcomes -Measures of disease frequency (incidence & prevalence) -Measures of association (absolute difference measures & relative difference measures; relative difference measure types are 'risk ratio & 'odds ratio') -Measures potential impact
How to interpret results of RULA?
Final Score: 1 or 2: Acceptable 3 or 4: Investigate further 5 or 6: Investigate further and change soon 7: Investigate and change immediately Low scores have lower risk of injury development, high scores, higher risk Low scores don't guarantee that ergonomic hazards are not present High scores don't guarantee that severe problems exist Developed to detect work postures that require further attention
What are OSHA's basic screening tools?
Five Risk Factors, Task descriptions, and relevant muscle groups
What are standing aids?
Footrests, anti-fatigue mats, footwear, & Compression hosiery
What are the components of the NIOSH Workplace evaluation?
General Ergonomic Risk Analysis Checklist, general checklist focusing on primary job activities, checklists for specific tasks or workplace conditions (workstation layout, task analysis, hand tool analysis, materials handling, computer workstation), very comprehensive coverage of diverse risk factors, and Yes/No format (hierarchical organization of some items)
What are the pitfalls and precautions?
Healthy worker survivor effect (error created by systematic differences between those selected and not selected for study; may make exposures appear protective; worker with longer exposures may appear to have lower disease rates) Sampling Variability (Random occurrence (type I and type II errors); less likely with larger sample sizes) Confounding (confusion of magnitude of association because of another variable; confounding variables are associated with both outcome and exposure) Hawthorne Effect - a myth that is often mistaken as a fact - there is no standard definition
What is the fundamental assumption of method assessment in regards to epidemiology?
Human disease and causal factors are not distributed at random Those who develop disease tend to be exposed to risk factors more than those who don't develop disease Epidemiological research methods approximate that statistical (probabilistic) effects of potential risk factors (exposures) on disease outcomes
What is the purpose of the strain index?
Identify jobs (not worker) with high risk for upper extremity morbidity
Identify shoulder muscles
Infraspinatus, deltoid, trapezius, and supraspinatus
What are the six multipliers of the strain index?
Intensity of exertion, duration of exertion, exertions/min, posture, speed of work, and duration/day
What can be said about toe clearance?
Lack of TC, cause to stand further away
Goals of workplace design & layout
Maximize performance and minimize hazards: -Provide reach capability (Anthropometry) -Minimize motion times and error rates (Work measurement; e.g. 30% time increase when working overhead) -Provide force capability (Strength data and models) -Minimize postural stress and fatigue
What is the Ovako working posture analysis system (OWAS)?
Method for evaluation of postural load during work Simple and systematic classification of work postures combined with observations of work tasks Allows for quick identification of work "risk" phases of work
Time task requirements
Minimize postural constraint -suggest rest breaks and stretching -develop duties that require operator to leave workspace -Facilitate a dynamic workspace (Change layout of components, encourage use of different postures, and stability balls)
Seating Objectives for the spine
Normal spine curve such as in standing (Cervical and lumbar lodossis, and thoracic kyphosis) -Distribution and transmission of forces -Low lumbar external bending moments -Low internal (intra-discal tissue pressure) -Mobility
Tools for checklists?
OSHA's basic screening tool, NIOSH workplace evaluation methods, OWAS, REBA, RULA, and strain index
Why do we use checklists?
Overall picture of which jobs need analysis; organized means for obtaining overall indication of presence or absence of risk factors
What are risk factors?
Personal or job-related attributes that are associated with (and may cause) increased risk of injury Examples: High force, repetition, vibration, etc
What is point prevalence?
Prevalence at one point in time
What are components of seated work guidelines
Primary visual display angle, viewing distance, secondary visual display angle, work surface height, top of keyboard height, top of primary display height, maximum height of workstation
What does measurement provide in seat design?
Provide function, comfort, and hopefully health
Issues with static work
Quite simply, we aren't built to stand, or sit still for great periods of time Static postures, whether sitting or standing, should be minimized as much as possible (duration & biomechanical load) -If unavoidable, the workstation should be optimized to reduce the amount of postural stress
A well designed seat should
Reduce postural stress/fatigue by: -distributing or supporting body segment weights -maintaining lordotic spinal curve -allowing user mobility Reduce Circulatory impingement Allow or support fine work
What are the risk factors evaluated for RULA?
Repetition, force, and non-neutral postures
What are the 5 risk factors?
Repetition, force, non-neutral postures, contact stress, and vibration
Workstation Design: Topics
Seated work Standing work General workstation and machine control guidelines -Joint moments and postural maintenance -localized muscle fatigue
Components to static anthropometry?
Shapes and clearances, accommodate population variances, design for mean or design for extremes, functional measures (reach analysis)
Other considerations in seat design
Stability - chairs should have five legs instead of four for stability Adjustability for posture - other work station design criteria Height adjustability - problems with nerve compression & circulation restrictions Too high -> pressure behind knee Too low -> Body weight not distributed over back of legs
Steps of RULA
Step 1 - Observe and select posture(s) to assess -Observe the postures adopted while performing the tasks -Select specific posture(s) for assessment (e.g., longest held or most deviated) -Assessments taken at set intervals can yield proportions Step 2 - Scoring and recording the posture(s) -Score the posture of each body part using tables/software -Review the scoring and make any adjustments, if required, for force levels and muscle use Step 3 - Grand Score and Action Level -Grand score computed from earlier information, using table/software -Action Level determined from Grand Score (acceptable immediate changes required)
Where can you find information on seated anthropometry?
Tabularized values and population distributions are readily available (static)
What is the strain index?
The Strain Index (SI) is a dimensionless measure of the risk or likelihood of developing a WMSD, and is derived through a semi-quantitative job analysis of six task variables (risk factors) The underlying 'model' is multiplicative with respect to the six task variables
Which forces at on the body while seated in a chair?
The main force on the body is transferred from the seat, some force from the floor, the backrest, and the armrests.
What is the incidence rate?
The number of new cases/persons at risk Ex: 10 CTD/1000 workers
What happens to the pelvis when sitting upright and relaxed?
The pelvis rotates posteriorly and the spine curve flattens
What are general ergonomics checklists used for?
The preliminary investigation for working conditions and is derived from a variety of sources
What is the basis of RULA?
The upper extremity is treated separately from the torso, neck, and lower extremity (captures influences of torso, neck, and leg constraints on UE postures) No previous experience needed Judgments are based on postures and measurements, not individual interpretation
Limitations of RULA?
This system provides a guide for further action Typically, problems identified require a more detailed investigation Can be time consuming for complex (or non-cyclic) jobs Practice and training/learning required for use Posture category distinctions are somewhat arbitrary Does not explicitly account for frequency and duration Limited number of studies on reliability and validity
How do you categorize scoring with REBA?
Torso, legs, and neck scored along with load/force estimates (Table 1)—Score A Upper and lower arm, wrist, and coupling (Table 2)—Score B Use Table 3 to find the intersection of Scores A and B, add activity score (changes in motion) Use Table 4 to identify the action level
Questions to ask when selecting a checklist/tool?
What is the goal? What is the coverage or application area? What is included? What is excluded? What is generated? Usability? Interpretation of results (when is action indicated)?
When are deltoid and trapezius activity needed?
When chair height is low or table is high
Lordotic lumbar curve
Without lumbar support during sitting, the normal lordotically curved spine flattens (straightens) to a more kyphotic shape
What body areas addressed with RULA?
Wrists/forearms, elbows, shoulders, neck, and trunk
What is incidence (Measures of disease frequency)?
number of new incidences (cases) of a disease in a population during a specified time period
Footrests enable what?
shifting of loads - eliminate constrained work postures